If the gearing in an internal combustion engine is so arranged that the engine can always operate at maximum capacity, a very economical operation results. To accomplish this, either a continuously variable gearing extending over a relatively large range is required or, for gears which are operable in steps, a fifth or sixth gear (a so-called overdrive gear) must be furnished. For a short term view, the overdrive solution is preferrable since continously varying gears have a lower efficiency. While the use of an overdrive gear in cross country driving can decrease the fuel consumption by as much as 15%, it does require much additional shifting. While planetary gearing as is generally utilized in the known systems with overdrive gears, is much easier to shift and allows the shifting to take place without output power interruption, it is also much more expensive than the normal spur gear system. Even the change from a three gear planetary gearing to a four gear planetary gearing results in much increased expense. Therefore a number of gear control systems have been developed which are semiautomatic, utilized spur gear trains and tolerate the output power interruption.
A number of systems are known in which the shifting was made easier for the driver by allowing the clutch to be operated with the gear shift lever. In these systems touching of the gear shift lever activated an electrical contact which in turn activated a clutch control system. In other systems, the shifting process itself releases the clutch control. The disadvantage of these systems is that all gears must be activated manually which is a great inconvenience to the driver especially when a fifth and sixth gear are provided.
Fully automatic gear shift systems for spur gear trains with power interruption are also known. In U.S. Pat. No. 3,433,101 an electrical control of the automatic gear shifting in a motor vehicle is shown in which the release of the clutch causes the gears to be shifted in a sequence depending upon the vehicle speed. After the correct gear ratio has been set, the clutch is automatically reengaged. In this system a generator furnishes an output voltage which varies as a function of vehicle speed. The output voltage is applied to a plurality of threshold stages, each having a different threshold, each being connected to a solenoid valve which, when energized, causes a corresponding gear to be engaged. The disadvantage of this arrangement is that the gear shift is completely pre-programmed and no gear shift at all can take place under the driver's control. Further, a hydraulic apparatus must be furnished to operate the throttle valve during the gear shift, since otherwise the speed of the engine would become excessive while the gears are being shifted.
Another system is disclosed in DT-PS No. 908,942, which utilizes an auxiliary gear train. The auxiliary gear train comprises planetary gearing which is controlled by a control signal which in turn depends upon the speed with which the gas pedal is activated by the driver. However, this system does not constitute an automatic gear control system since only the addition of an auxiliary gear is controlled and the actual operating conditions of the motor vehicle such as the vehicle speed, the throttle position, etc. are not taken into consideration at all.
In DT-PS No. 1,555,170, a system is described in which a so-called "kick down" command is derived from the speed with which the gas pedal is activated. However, only a shift-down command can be initiated by this system. It is thus unsuitable for use of overall control of an automatic gear shift system.
U.S. application Ser. No. 407,975, filed Nov. 11, 1964, describes an arrangement similar to that disclosed in DTP 1,555,170 wherein the down-shift command is derived when the speed with which the pedal is activated exceeds a predetermined threshold speed. This arrangement is not suitable for the reasons given above insofar as the control of an automatic gear shift apparatus is concerned.